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Solar Hot Water — It Just Makes Cents

A few winters ago my brother and I fixed the solar hot water system belonging to some friends of ours. The piping on the roof had sprung a leak and needed a small repair, unfortunately in the middle of a snowstorm.

The longevity of this system (more than 20 years) and the tremendous cost and energy savings enjoyed during that time by the home’s residents truly amazed me. But before I go into those details, let me first tell you a bit more about this system.

This solar hot water system is a simple two-panel thermosiphon system, meaning that it uses natural convection to circulate the liquid through the system, eliminating the need for a pump and thereby saving energy. The system was installed more 20 years ago to be the primary hot water heating system for this two-bedroom home in Eldorado. At some point, a traditional hot water heater was installed and the solar became the backup system, feeding hot water to the heater’s cold water inlet.

This system will typically feed 120-130° F water directly into the heater on the coldest winter days, thereby supplying most of the hot water required for this two-person household. On sunny days in the spring, summer and fall, water from the panels may exceed 160° F and supply all the hot water needs. The system has no pump and no moving parts so there is little to go wrong or maintain.

Allan showed me a copy of the original brochure and I was astounded. Twenty years ago, the original system cost $895 for a single panel system — installed — and included a used hot water heater. Yes, $895 installed! Each panel contains more than 37 gallons of storage. Altogether, the two-panel system plus hot water tank holds more than 100 gallons of hot water — far more than most traditional hot water systems installed today.

For two decades, this system has been supplying seemingly endless hot water to Allan and Francesca as well as to the home’s prior residents. Factoring in inflation, this $895 system would cost $2,500 today. That’s a real bargain considering that a typical system today costs around $4,500 for materials only.

But as much as I was impressed by the simplicity and longevity of this system, it was the energy savings that really got my attention.

Assuming that this system has been running continually since installation, it has supplied well over 100,000 gallons of hot water to this home. A typical household will use about 18-20 gallons of water per day. So assuming 20 years * 365 days * 20 gallons, that’s 146,000 gallons! Taking a conservative estimate, let’s assume that the system only supplied 75% of the home’s hot water due to cloudy days, that’s still about 90,000 gallons of free hot water! And it’s still providing hot water, at no additional cost, twenty years later!

According to a study done by the Department of Energy, residential water heaters currently account for roughly 24% and 11% of a home’s total gas and electricity consumption respectively. Our friends’ traditional water heater is electric. With their solar system providing most of their hot water needs, the electricity consumption of their hot water heater has dropped by about two-thirds, a substantial energy and cost savings.

Energy is measured in units called BTUs (British thermal units). One BTU is the amount of energy needed to heat one pound of water one degree Fahrenheit. Assuming that unheated municipal water enters the home at around 55° F and solar heated water is 120° F, then the difference is 75° F. Assuming the home uses 20 gallons a day, that equates to more than 91,000,000 BTUs saved annually!

Because electricity rate have fluctuated over the decades, it is difficult to calculate precisely how much it would have cost to heat that volume of water 75° F using electricity. But if we apply today’s average electricity rate of $0.1179 per kWh (source: U.S. Energy Information Administration), and 1 kilowatt-hour (kWh) = 3,413 BTUs, the BTUs provided free of charge by the sun has conservatively saved this household $3,100 over the years. In other words, it has paid for itself several times over.

Besides being an excellent investment for the owners, it has helped reduce pollution and save thousands of gallons of water. A study done by the U.S. Geological Survey finds that 48% of all the water consumed in the U.S. goes to power plants. Depending on the method used to generate the electricity, conventional power plants can use between 0.4 to 0.6 gallons per kWh generated. According to the U.S. Energy Information Administration, the national average output rate for coal-fired electricity generation was 2.095 pounds CO2 per kWh. So in the last twenty years, this system has saved between 10,000-15,000 gallons of water and reduced air pollution by more than 27 tons of CO2.

Not bad for a single solar hot water system. It has saved thousands of dollars, helped the environment, and brought many, many years of personal satisfaction to its owners who know they are doing the right thing. It has been good for the homeowners and good for the community. What a truly amazing system. Just imagine the impact if each of us installed a system.






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